Charged particle beam focusing system



Sept. 1, 1959 MlCHlAKl n'o CHARGED PARTICLE BEAM FOCUSING SYSTEM FiledApril 23, 1956 FIG. 2.

FIG. 3.

INVENTOR M/CH/AK/ r0 ATTORNEY United States Patent G CHARGED PARTICLEBEAM FOCUSING SYSTEM Application April 23, 1956, Serial No. 579,841

6 Claims. (Cl. 3153.6)

This invention relates to focusing means for beams of charged particlesand is particularly applicable to electron discharge tubes using arelatively long electron beam which must be focused throughout itslength, such as'is used in traveling wave tubes and the like. Theinvention may be termed a quasi-static focusing system as distinct fromthe electrostatic and magnetic focusing systems now in use which uses adirect voltage or current or permanent magnets.

Since the principles of alternate concentration type of focusing forbeams was made public in 1948 several methods by which flow of chargedparticles may be focused, either electrostatically or magnetically, withfields which periodically change in polarity and intensity, have beenproposed. For all of these systems however a completely stabiliseddirect current source has been necessary except in cases where permanentmagnets have been used in the magnetically focused beam systems.According to the present invention, an alternating current may be usedto achieve the focusing from any multiple phase source such as thenormal 50-60 cycle threephase supply. Although the normal commercialsupply is generally preferable because it is convenient to use, theinvention is applicable for any multiple-phase lowfrequency supplyhigher or lower than the commercial supply. Also any polyphase powersource, such as six or twelve phase systems may be used.

According to a feature of this invention use is made of the relativelylong period of the A.C. source supply with respect to the time requiredfor the charged particles forming the beam to traverse the area toachieve the effective steady static field condition during traversaltime of these charged particles.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionwill be best understood by reference to the following description of anembodiment of the invention taken in conjunction with the accompanyingdrawings, in which;

Fig. 1 is a diagram illustrating an embodiment of the invention used inexplaining the operation of the system;

Fig. 2 is a schematic diagram of a traveling wave tube using thefocusing arrangement of the type illustrated in Fig. 1, and

Fig. 3 illustrates an alternative type of focusing arrangementincorporating the principles of this invention.

The principle of the invention may be understood from the followingdescription made with reference to Fig. 1. In this figure there is shownthree helical coils 2, 3 and 4. These coils being of equal diameter andhaving the turns of the respective coils equally spaced lengthwise onefrom the other. These coils also surround an electron beam indicatedat 1. To the respective helices 2, 3 and 4 are supplied three-phasevoltages which may be expressed by:

E, cos (0 +wt) E0 COS E COS (4/31r|0 +wl) respectively. Where E is themaximum voltage 0 is the initial phase angle and w=21rf, and f is thefrequency of the energy.

Helices 2, 3 and 4 are insulated from DC. flow at the lower audiofrequenciesbut may be coupled at opposite ends to sources of ultra highfrequency. If the time for passage of a charged particle through thelength of the helices is short compared to the period of the alternatingpotentials applied to the helices, the potentials on the helices may beconsidered as remaining substantially constant during the travel time ofsuch a charged particle. This fact can be derived by computation but, inview of the order of magnitudes of the relative time periods of the waveand the passage of time of elements or other charged particles, suchcalculations need not be included for an understanding of the invention.

In electron beam tubes such as traveling wave tubes or backward wavetubes, the electron accelerating voltage generally ranges betweenseveral hundreds to several thousands of volts while the length of thehelix is at most in the order of a fraction of one meter. Thus the timefor passage of a charged particle throughout the length of the helixwill be in the order of one hundredth of a micro-second, whereas theperiod of the alternating current may be in the order of two hundredthsof a second. It will thus be clear that the electric field due to anA.C. voltage may be considered as static during the passage time of acharged particle through the length of the helix as the time is tooshort for any appreciable change to occur. Thus the distribution of theelectric field along the length of the helices of Fig. 1 will besubstantially as shown in the dotted lines 5, at any instant of timeduring which the charged particles are passing through the length of thehelix. The electric field will be substantially the same as that whichwould be generated between the turns of helix 2 and an equivalent helix6 positioned intermediate the turns of helices 3 and 4. Accordingly,electrons traversing the length of the helix will be subjected toalternate converging and diverging forces. It can thus be assumed thatan electron as indicated by 7 would follow substantially a sine wavecurve as indicated resulting in a substantially parallel flow of theelectrons thorughout the length of the helices. This effect cannot beachieved by use of a single helix excited by alternating current but isreadily obtained with a polyphase supply such as the three phase supplydescribed in connection with Fig. 1.

An example of the application of the principles de scribed in connectionwith Fig. l is illustrated in the schematic arrangement of Fig. 2. Inthis figure an electron beam from a cathode 11 is projected through theturns of helices 2, 3 and 4 to a target electrode 15. During the travelof the beam particles through the helices a high frequency wave appliedto the helices 2, 3 and 4 from an input circuit 16 will be amplified byinteraction of the electrons of the beam and the high frequency wave sothat the high frequency amplified Wave may be taken out at 17. The threephase supply for helices 2, 3 and 4 may be furnished from thesecondaries 8 of a three phase transformer. As illustrated a Yconnection of secondaries 8 is shown and the common point 9 of thesesecondaries may be connected to a positive terminal of a DC. source 12.Thus the helices may be used as the initial accelerating electrodes forthe electron beam at the same time they are being used for the focusingof the electron beam and as the delay line for the high frequency energyto be amplified. Another D.C. source 13 is shown connected to thecollector electrode 15.

If any modification of the diameter of the wires used in the helices isdesired for the purpose of suppressing .5 high frequency propagationmodes of the waves to be amplified, the variations in the focusing fieldproduced thereby may be compensated by suitable impedance networksconnected at;the input leads 14 from the three .phase supply to thehelices.

In the description of Figs. 1 and 2 the invention has been applied tothe structure of a traveling wave tube utilizing helices for effectingretardation of the waves three sets of rings 18, 19 and 20 successivelypositioned substantially in the same manner as the successive turns ofthe various helices of Fig. l. Thesefocusing electrodes may then besupplied with polyphase voltages over line 14 from any suitable sourcesuch as shown in Fig. 2.

While the invention has been described in connection with the control ofelectrons, it is clear that the principles of the invention apply to anyforms of beams of charged particles, such as ions or other elementaryparvticles.

By the useof this invention it is not necessary to use a magnetic fieldfor focusing a flow of charged particles and thus the weight of thestructure can be greatly reduced. Furthermore, complete stability of theelectric source is not required as in the case of DC. electrostaticfocusing. So that a much simple source of focusing power can be made.

While the principles of this invention have been described in connectionwith specific apparatus, it is to be clearly understood that thisdescription is made only by way of example and not as a limitation tothe scope of the invention as set forth in the objects thereof and andin the accompanying claims.

, What is claimed is:

1. A beam focusing system for focusing a beam of charged particles,comprising means for producing a beam of charged particles, a pluralityof conductor elements positioned symmetrically and coaxially of thebeam, a source of polyphase energy having a period long with respect tothe time for passage of particles of said beam through said conductorelements, and means for applying energy of different phases from saidsource to respective successively positioned of said conductor elements.

2. A beam focusing system according to claim 1, wherein said conductiveelements comprise successive turns of helical conductors, said heliceshaving substantially equal pitches and diameters, and being positionedto have equal spacings between adjacent turns.

3. A beam focusing system according to claim 2, wherein said beam is abeam of electrons.

4. A traveling wave tube incorporating a beam focusing system accordingto claim 3, wherein said electron beam is included in a traveling wavetube, further comprising means for applying a high frequency wave tosaid helices for interaction with said electron beam.

5. A beam focusing system according to claim 1,

wherein said polyphase source is a three phase source,

and said plurality of conductor elements comprise the turns of threehelices, positioned to have equal spacings between adjacent turns.

6. A traveling wave tube incorporating a beam focusing system accordingto claim 5, wherein said helices comprise the high frequency energyretarding means of a traveling wave tube, further comprising an electronsource and a target electrode positioned at opposite ends of saidhelices to produce an electron beam coaxially of said helices formingsaid beam of charged particles, means for applying high frequency energyto one end of said helices for interaction with said electron beam, andmeans for extracting amplified high frequency energy at the other end ofsaid helices.

References Cited in the file of this patent UNITED STATES PATENTS2,218,725 Schroeder Oct. 22, 1940 2,260,851 Bruche Oct. 28, 19412,373,837 Linder Apr. 17, 1945 2,616,062 Charton Oct. 28, 1952 2,725,499Field Nov. 29, 1955 FOREIGN PATENTS 157,198 Australia June 23, 1954

